1. Field of the Invention
The present invention relates to an optical signal quality monitoring system for monitoring the signal-to-noise ratio (S/N) of digital optical signals having different bit rates, transmitted in an optical fiber transmission network.
This application is based on Patent Applications Nos. Hei 9-330553 and Hei 10-229659 filed in Japan, the contents of which are incorporated herein by reference.
2. Description of the Related Art
In the network hierarchical structure SDH (Synchronous Digital Hierarchy) which was internationally standardized in the 1990s, a parity check called xe2x80x9cbit interleave parity (BIP)xe2x80x9d is executed between repeaters (this case is called xe2x80x9cBIP-8xe2x80x9d and is explained later) and also between line terminal multiplexers (this case is called xe2x80x9cBIP-Nxc3x9724xe2x80x9d), thereby identifying an erroneous section and obtaining a signal for switching and activating operations. Here, xe2x80x9cNxe2x80x9d (an integer) relates to the level of the multiplex. With the basic symbol xe2x80x9cSTM-1xe2x80x9d indicating 156 Mbit/s, xe2x80x9cSTM-Nxe2x80x9d means the level obtained by multiplying the above level by N. The cases of N=1, 4, 16, and 64 are internationally standardized. In addition, xe2x80x9cBIP-Mxe2x80x9d indicates a parity check for every M bits, and a set of M bits for checking are obtained. At the transmission side, the parity check of parallel M bits of a signal included in a frame is executed, and the checked bits are stored in the next frame and are transmitted with the main signal. At the receiving side, a similar parity check is executed, and a transmission error is detected by collating the checked bits with the above checked bits stored in a specified area in the next frame.
FIG. 18 shows an example of conventional systems for measuring a (bit) error rate. In the figure, a portion of an optical signal through a transmission path is separated and extracted by optical coupler 51-1. The extracted portion is amplified by optical amplifier 52 and is further separated into two portions by optical coupler 51-2. One portion of these two portions is input into clock-extracting circuit 53 so that a clock signal of frequency f0 (i.e., xe2x80x9cclock f0xe2x80x9d) is extracted. The other portion separated by optical coupler 51-2 is input into receiving circuit 54, an output thereof further input into error-rate detecting circuit 55 which consists of a frame detecting circuit, parity checking circuit, and collation circuit. The receiving circuit 54 and error-rate detecting circuit 55 are operated in accordance with the above clock f0 extracted by clock-extracting circuit 53, and the error rate of the optical signal is measured. Here, the clock-extracting circuit 53, receiving circuit 54, and error-rate detecting circuit 55 must have specific structures corresponding to the bit rate of the target optical signal. That is, in order to perform error-rate detection corresponding to plural kinds of bit rates, plural circuits which respectively correspond to the different bit rates are necessary, and thus the error-rate detection cannot be executed using a single circuit based on the conventional technique.
Generally, the error rate of a signal is directly measured in order to evaluate a transmission system. However, if the error rate is very low in this method, a long measuring time is necessary and thus the measuring efficiency is low.
Therefore, regarding a transmission system for receiving binary digital signals, a method for estimating an error rate was presented, in which according to a tendency of error rates obtained when the threshold for a decision circuit is shifted, the error rate at the optimal operating point is estimated (refer to Reference 1: N. S. Bergano, et al., xe2x80x9cMargin Measurements in Optical Amplifier Systemsxe2x80x9d, IEEE Photonics Technology Letters, Vol. 5, No. 3, pp. 304-306, March, 1993). FIG. 19 shows a relevant eye diagram of an optical signal and an amplitude histogram indicating light intensity. The threshold of the decision circuit is changed at the time to when the eye-diagram opening is maximum (i.e., the decision point), thereby discriminating between the xe2x80x9cHighxe2x80x9d (or xe2x80x9c1xe2x80x9d or xe2x80x9cMARKxe2x80x9d) level and the xe2x80x9cLowxe2x80x9d (or xe2x80x9c0xe2x80x9d or xe2x80x9cSPACExe2x80x9d) level in the binary data transmission, and measuring each error rate.
In practice, a measuring system as shown in FIG. 20 is constructed, which consists of clock-extracting circuit 53, photoelectric converter 56, and electrical signal processing means 57, and the Q-factor (as an evaluation index) corresponding to the S/N is calculated based on dependency of the error rate on the threshold. In more detail, a portion of an optical signal extracted from a transmission path is converted into an electrical signal by the photoelectric converter 56, and this electrical signal and a clock (signal) extracted by the clock-extracting circuit 53 are input into electrical signal processing means 57 such as a sampling oscilloscope, so that an eye diagram and an amplitude histogram as shown in FIG. 19 are obtained.
At time t0 when the eye-diagram opening is maximized, with signal amplitude (such as a voltage) xcexc(t0), standard deviation "sgr"1(t0) of noises at the xe2x80x9cMARKxe2x80x9d level, and standard deviation "sgr"0(t0) of noises at the xe2x80x9cSPACExe2x80x9d level, the Q(t0) (i.e., the Q-factor at t0) is represented as follows:
Q(t0)=xcexc(t0)/("sgr"1(t0)+"sgr"0(t0))xe2x80x83xe2x80x83(1)
On the assumption of a Gaussian distribution of the amplitude of noises, in a low error-rate range, the following relationship between error rate P and the Q-factor is obtained:
P=(1/(Q(2xcfx80)xc2xd)) exp (xe2x88x92Q2/2)xe2x80x83xe2x80x83(2)
Therefore, if the Q-factor can be determined, the error rate can be estimated.
However, in the conventional Q-factor measuring system, an optical signal is converted into an electrical signal, and the waveform of the converted signal is sampled so as to determine the Q-factor. Therefore, the possible bit rate of the optical signal is limited to approximately 40 Gbit/s, depending on the range or processing speed of the photoelectric converter and the electrical signal processing circuit.
In addition, the Q-factor at the time when the eye-diagram opening is maximum is measured; thus, the system cannot monitor plural digital optical signals having different bit rates.
Furthermore, in the conventional Q-factor measuring system, a portion of an optical signal to be monitored must be extracted from the transmission path. Therefore, power loss due to the separation of the optical signal transmitted in the transmission path is generated, thereby degrading the S/N.
Accordingly, the present invention has an objective to provide a system for monitoring the quality of optical signals which are transmitted in an optical fiber transmission network and which have different bit rates, where the S/N of each optical signal can be monitored.
The present invention has another objective to provide a system for monitoring the quality of optical signals having bit rates of a few dozen Gbit/s or more.
The present invention has another objective to provide a system for monitoring the quality of optical signals, by which the effect on the S/N of each optical signal transmitted in a transmission path can be reduced.
Therefore, the present invention provides an optical signal quality monitoring system comprising:
sampling means for sampling an optical signal having a bit rate Nxc2x7f0, that is, N times as much as the basic clock frequency f0 where N is a natural number, by using a pulse repetition frequency f0/n1xe2x88x92xcex94f or f0/n1+xcex94f where n1 is a predetermined natural number and the pulse repetition frequency slightly differs from f0/n1 by xcex94f; and
electrical signal processing means for determining an amplitude histogram of the light intensity of the optical signal based on the results of the sampling, and regarding the sampling points which constitute the histogram, the processing means extracting a set of higher-level points and a set of lower-level points and calculating a ratio of a difference between an average level of the set of higher-level points within a predetermined period and an average level of the set of lower-level points within a predetermined period, to the sum of standard deviations of both sets within each predetermined period, the calculated ratio being a coefficient of the S/N, so as to examine the quality of the optical signal based on the coefficient.
According to this structure, it is possible to sample an optical signal and obtain an amplitude histogram, and to monitor a temporally-averaged coefficient (i.e., a Q-factor) relating to the S/N based on statistically-processed amplitude values of the sampling points of the histogram. That is, the quality of any optical signal can be monitored independantly of the bit rate of the optical signal.
Preferably, the sampling means comprises:
sampling optical pulse generating means for generating a sampling optical pulse train having the pulse repetition frequency f0/n1xe2x88x92xcex94f or f0n1+xcex94f which slightly differs from f0/n1 by xcex94f;
an optical multiplexer for multiplexing the optical signal and the sampling optical pulse train;
a nonlinear optical material for generating and outputting a cross-correlation optical signal caused by a nonlinear optical effect by using the optical signal and the sampling optical pulse train input from the optical multiplexer so that the optical signal is sampled by the sampling optical pulse train; and
a photoelectric converter for converting the cross-correlation optical signal into a cross-correlation electrical signal, and
in the monitoring system, the electrical signal processing means determines the amplitude histogram based on the cross-correlation electrical signal.
According to this arrangement, optical sampling with respect to an optical frequency range is performed using optical means; thus, the quality of optical signals having ultra-high speed of a few dozen Gbit/s or more can be examined, which is difficult using the conventional technique. That is, the quality of optical signals having different bit rates within a wide range from 1 Mbit/s to 100 Gbit/s can be monitored using a single monitoring system (or circuit).
For quality monitoring of optical signals having bit rates of approximately a few dozen Gbit/s or less, the coefficient (i.e., Q-factor) of the SAN may be monitored using a structure for performing electrical sampling.
Typically, the sampling means comprises a photoelectric converter for converting the optical signal into an electrical signal, timing-clock generating means for generating a timing-clock signal having the pulse repetition frequency f0/n1xe2x88x92xcex94f0/n1+xcex94f which slightly differs from f0/n1 by xcex94f, and electrical-sampling means for performing sampling of levels of the electrical signal by using the timing-clock signal.
In an optical fiber transmission network which uses optical amplifiers as repeaters, the bit rate of an optical signal can be flexibly selected. The monitoring system according to the present invention can be suitably applied to such a network. In addition, by selecting a suitable bit rate within a possible range in which the sampling optical pulse generating means can normally operate, the present invention can be applied to a transmission path for a signal, based not only on the network hierarchical structure SDH but also on another network hierarchical structure PTH (Presynchronous Digital Hierarchy).
It is possible for the optical multiplexer to comprise polarization-splitting and multiplexing means for splitting the optical signal into two orthogonal polarization components Psig. p and Psig. s and also splitting the sampling optical pulse train into two orthogonal polarization components Psam. p and Psam. s, and for polarization-multiplexing orthogonal components Psig. s and Psam. p with each other, and also polarization-multiplexing orthogonal components Psig. p and Psam. s with each other, and outputting the multiplexed signals from two output ports, and that regarding the dual polarization-multiplexed signals, cross-correlation optical signals are respectively generated and these signals are then synthesized. Accordingly, it is possible to remove polarization dependency in which the power of the cross-correlation optical signal changes in accordance with the polarization state of the input optical signal.
It is also possible to insert the optical multiplexer and the nonlinear optical material into the transmission path through which the optical signal is transmitted; to multiplex the optical signal from the transmission path and the sampling optical pulse train output from the sampling optical pulse generating means by the optical multiplexer and to input the multiplexed signal into the nonlinear optical material; with the monitoring system comprising wavelength-division demultiplexing means, inserted into the transmission path, for demultiplexing the optical signal and the cross-correlation optical signal output from the nonlinear optical material, and outputting the separated optical signal into the transmission path and outputting the separated cross-correlation optical signal into the photoelectric converter.
According to this arrangement, power loss of the optical signal transmitted through the transmission path can be reduced.